Bulletin 90-02: Loss of Thermal Margin Caused by Channel Box Bow
OMB No.: 3150-0011 NRCB No. 90-02 UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 March 20, 1990 NRC BULLETIN NO. 90-02: LOSS OF THERMAL MARGIN CAUSED BY CHANNEL BOX BOW Addressees: All holders of operating licenses (OLs) or construction permits (CPs) for boiling water reactors (BWRs). Purpose: The purpose of this bulletin is to request that addressees determine whether any channel boxes are being reused after their first bundle lifetime and, if so, ensure that the effects of channel box bow on the critical power ratio (CPR) calculation are properly taken into account. Background: As a result of information obtained at a meeting on fuel failures caused by dryout at a foreign BWR facility and at meetings with BWR fuel vendors, the NRC issued the attached Information Notice 89-69, "Loss of Thermal Margin Caused by Channel Box Bow," on September 29, 1989, to alert addressees to the potential problems associated with excessive channel box bow that could result in a loss of thermal margin. The main concern applicable to the U.S. BWRs at this time is that the channel box bow effect has not been taken into account in the CPR calculation for channel boxes in their second bundle lifetime operation. Discussion: Because channel box bow has a non-negligible impact on BWR thermal limits calculations, it is important that actions are taken to prevent the possibility of any fuel failures because of this effect. The staff acknowledges that the possibility of fuel failure as a result of channel box bow for channel boxes in the first fuel bundle lifetime is remote for U.S. BWRs. However, channel boxes in use for a second bundle lifetime may be severely bowed with a loss of CPR margin of up to 0.20 delta-CPR. Therefore, the staff has concluded that all addressees that currently use channel boxes for a second bundle lifetime should inform the NRC of the number and disposition of such channel boxes in the core, and describe the actions taken to assure compliance with the technical specification CPR limits. Because of the new information about channel box bow, the staff expects that reuse of channel boxes in future core reloads will be addressed as an unreviewed safety question in reload core safety evaluations. 9003130012 . NRCB 90-02 March 20, 1990 Page 2 of 3 Addressees who use channel boxes for only a single bundle lifetime are expected to take into account the effects of channel box bow in analysis supporting the next fuel reload application. This can be done by either the use of an approved methodology that takes the effects of channel box bow into account in the CPR calculations, or by the use of a bounding value of delta-CPR. Preliminary results have been obtained by both General Electric Company (GE) and Advanced Nuclear Fuels Corporation (ANF) in their proposed methodologies. It is expected that the delta-CPR caused by channel box bow would not exceed 0.04 for D-lattice plants and would not exceed 0.02 for C-lattice and S-lattice plants. These results are based on the maximum expected channel box bow for channel boxes that are used for only a single bundle lifetime. Because of the increased rate of channel growth observed at the foreign BWR facility after about 50 to 60 GWd/MTU burnup, the delta-CPR effect is expected to be much larger for channel boxes being used in their second bundle lifetime. However, GE and ANF have not provided recommendations for the use of second bundle lifetime channel boxes. Both GE and ANF have provided the NRC with reports describing their methodology for incorporating the effects of channel box bow in their CPR analysis. The GE report was submitted to the NRC by letter dated November 15, 1989, and is entitled "Effect of Channel Bow on Margins to Core Thermal Limits in BWRs." The ANF report was submitted to the NRC by letter dated November 30, 1989, and is entitled "Critical Power Methodology for Boiling Water Reactors - Methodology for Analysis of Assembly Channel Bowing Effects," ANF-524(P), Revision 2, Sup-plement 1. These two methodologies are presently under staff review, which is expected to be completed by May 1990. In the interim, the staff will consider approval of proposed plant- and cycle-specific applications of these new methodologies, if it can be demonstrated that the new methods are conservative in comparison to previously approved methods and are being correctly implemented. In summary, all BWR licensees are expected, in evaluation of the design and operation of reload cores, either to implement an approved methodology or to implement an acceptable bounding value of delta-CPR, as needed, to assure that the effects of channel box bow on CPR limits are taken into account to assure compliance with technical specification CPR limits. Requested Action: All BWR licensees that currently use channel boxes for a second bundle lifetime should verify that current CPR technical specification operating and safety limits are met. Reporting Requirements: Within 30 days of receipt of this bulletin, all holders of OLs for BWRs that currently use channel boxes for a second bundle lifetime shall advise the NRC of the number of such channel boxes and their disposition in the core. In addition, the response shall describe the methods, and the associated data base, used to account for the effects of channel box bow during the second bundle lifetime use of channel boxes to ensure conformance with the CPR technical specification operating and safety limits. . NRCB 90-02 March 20, 1990 Page 3 of 3 The written reports required above shall be submitted under oath or affirmation pursuant to Section 182 of the Atomic Energy Act of 1954, as amended (42 U.S.C. 2232), and shall be addressed to the U.S. Nuclear Regulatory Commission, ATTN: Document Control Desk, Washington, D.C. 20555. In addition, a copy shall be submitted to the appropriate Regional Administrator. Backfit Discussion: The action requested in this bulletin ensures licensees' compliance with their technical specifications as required under 10 CFR 50.36(c)(1)(i)(A). The safety limit CPR comes under this regulation. A known deficiency in current calculation of the CPR could result in fuel failures because the operators are misled to believe they are in compliance with the technical specification operating CPR limit. The requested action was evaluated consistent with the provisions of 10 CFR 50.109 and found to be covered by the provisions of paragraph (a)(4)(i). This request is covered by Office of Management and Budget Clearance Number 3150-0011 which expires January 31, 1991. The estimated average burden hours are 100 person-hours per licensee response, including assessment of the new recommendation, searching data sources, gathering and analyzing the data, and preparing the required letters. These estimated average burden hours pertain only to these identified response-related matters. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to the Information and Records Management Branch, Division of Information Support Services, Office of Information Resources Management, U.S. Nuclear Regulatory Commission, Washington, D.C. 20555; and to the Paperwork Reduction Project (3150-0011), Office of Management and Budget, Washington, D.C. 20503. If you have any questions about this matter, please contact one of the technical contacts listed below or the appropriate NRR project manager. Charles E. Rossi, Director Division of Operational Events Assessment Office of Nuclear Reactor Regulation Technical Contacts: Peter C. Wen, NRR (301) 492-1172 Daniel B. Fieno, NRR (301) 492-3236 Attachments: 1. Information Notice 89-69, "Loss of Thermal Margin Caused by Channel Box Bow" 2. 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